Zinc-coated steel with vapor-deposited aluminum overlay and method of producing same



Apnl 15, 1969 SHEPARD ET AL 3,438,754 ZINC-COATED STEEL WITHVAPOR-DEPOSITED ALUMINUM OVERLAY AND METHOD OF PRODUCING SAME Filed Feb.18, 1965 Zn coal/n9 ll /2 A/ Coating vapordepos/red I Sfee/ Sheer GeorgeA Shepard Afro/Hey United States Patent Office 3,438,754 Patented Apr.15, 1969 3,438,754 ZINC-COATED STEEL WITH VAPOR-DEPOSITED ALUMINUMOVERLAY AND METHOD OF PRO- DUCING SAME George A. Shepard, Parma, andCarl F. Brooker, Garfield Heights, ()hio, assignors to Republic SteelCorporation, Cleveland, Uhio, a corporation of New Jersey Filed Feb. 18,1965, Ser. No. 433,710 Int. Cl. 1521c 23/22; C23f 1.7/; B44d 1/14 U.S.Cl. 29-1835 12 Claims ABSTRACT OF THE DISCLOSURE A zinc-coated steelarticle with a vapor-deposited aluminum overlay is produced by applyingto a steel surface an adherent coating of zinc, and thereafter applyinga coating of aluminum to the zinc-coated steel surface by depositingaluminum vapor thereon.

This invention relates to the manufacture of zinc-coated steel articles.In particular, it is directed to methods of producing on a steel surfacea zinc coating having improved corrosion resistance, and also to theresulting articles, such as steel sheet carrying an adherentcorrosionresistant coating.

The coating of sheet steel and other steel articles with zinc, as byhot-dip galvanizing, electroplating or vapor deposition, is widelyemployed as a steel surface-treating operation. Zinc-coated steel sheet,meaning strip or other sheet of steel, whether cold rolled or hotrolled, has distinct advantages in many fields of use, particularly inavoiding or reducing corrosion or like deterioration of the steelsurface. As is well known, zinc protects steel galvanically, i.e., in acorroding medium the zinc will preferentially corrode; in any givencorroding medium the protection which the zinc affords the steel is afunction of its thickness.

Under severe conditions (as for example in exposure to salt spray) thezinc is rapidly consumed, and accordingly the protection afforded by thecoating against corrosion, in the case of conventional zinc coatings,may be relatively short-lived. In other words, under such conditionsconventionally zinc-coated steel articles may exhibit substantialcorrosion after comparatively brief periods of exposure.

An object of the present invention is to improve the resistance tocorrosion of zinc-coated steel articles and in particular to increasethe duration of effective protection against corrosion afforded by zinccoatings on steel surfaces. Another object is to provide methods ofproducing zinc coatings on steel, affording coatings having improvedcorrosion resistance. A further object is to provide zinccoated steelarticles having superior corrosion-resistant properties.

To these and other ends, the method of the invention in a broad sensecontemplates establishing an adherent coating of zinc on a steelsubstrate, e.g., in conventional mannet, and thereafter applying to thezinc-coated surface a further, thin coating or overlay of aluminum bydepositing aluminum vapor thereon. It is found that zinc-coated steelarticles thus produced, i.e., having a thin vapor-deposited aluminumoverlay, exhibit greatly enhanced resistance to corrosion and inparticular that the duration of effective protection against corrosionafforded by the zinc coating is very materially increased, as comparedto that afforded by conventional zinc coatings of equivalent thickness.This improvement in corrosion resistance is presently believed to be dueto some effect of the aluminum overlay in sealing the pores which existin zinc coatings and/ or otherwise serving to make the zinc surface lessactive thereby to retard the consumption of zinc under corrosiveconditions.

In the practice of the present method, the step of applying the zinccoating may itself, as stated, be entirely conventional. Thus, forexample, the zinc coating may be established on the steel substrate byhotdip galvanizing or by electroplating. In hot-dip galvanizing, thesteel surface, suitably cleaned, is immersed in a body of molten zinc toestablish on the surface a zinc layer that solidifies (after the surfaceis withdrawnfrom the zinc bath) as a coating adherent thereto.Electroplating of zinc involves immersing the steel surface (afterthorough cleaning thereof) in an appropriate electrolytic bath, e.g.,containing zinc sulfate, and passing direct electric current through thebath, to plate an adherent coating of zinc onto the surface. Theresultant zinc coating as prepared by either procedure may be whollyconventional in character, e.g., a conventional hot-dip galvanized orelectroplated zinc coating on steel, and may have a thickness (forexample) of the order of 20 to 1,500 microinches (one microinch being10* inch), or more depending upon conditions of operation of azinc-applying step. Both hot-dip galvanizing and electroplatingoperations as contemplated herein, including surface precleaning stepsand selection of conditions to produce a desired coating thickness, arewellknown in the art and accordingly need not be described in detail.

Following the application of the zinc coating, however produced, afurther coating, of aluminum metal, is applied to the zinc-coated steelsurface by vapor deposition. This vapor deposition step may be performedin accordance with conventional vacuum metalizing techniques fordepositing aluminum from vapor on a metal substrate. Thus, thedeposition may take place in a standard vacuum system for the vapordeposition of metals having a vacuum chamber, into which the steel ispassed after coating with zinc. To achieve desired adherence of thealuminum layer, the zinc-coated steel surface as introduced to thechamber must be thoroughly clean and dry; accordingly, between the stepsof applying the zinc and aluminum coatings, the surface may be cleanedor rinsed and dried, again in accordance with conventional proceduresfor preparing metal substrates for vacuum metalizing. For thevapor-depositing operation, the vacuum chamber is evacuated andmaintained at a relatively high vacuum, of the order of a half micron orless of mercury, and aluminum from a clean and uncontaminated source isevaporated, by heating the source, to produce aluminum vapor in thechamber. The vapor condenses or deposits on the zinc-coated surface asan adherent solid film of aluminum overlying the zinc coating, thethickness of this film being determined by the rate of vapor depositionand duration of exposure of the surface thereto.

One specific example of a vacuum-metalizing technique, suitable for usein depositing the aluminum coating in the present method, is thatdescribed in United States Patent No. 2,959,494 as employed to depositaluminum from vapor on a bare steel substrate. This procedure may beperformed in a vacuum system consisting of a vacuum metallizer and avacuum chamber. In the step of deposition of aluminum, the vacuumchamber is evacuated prior to aluminum deposition by a four stageprocedure which includes a first step of rough pumping to a vacuum of200 microns of mercury. The vacuum is then further reduced by the use ofdiffusion pumps. The residual atmosphere remaining after a pump-downmust not contain organic vapors. A high voltage discharge step providesgood adherence of the aluminum coating on the substrate; this iseffected while the vacuum chamber is being evacuated with the diffusionpumps. The surface of the substrate is degassed by a high voltagedischarge. After the high voltage discharge, the fourth step of thepump-down procedure carries the vacuum to about 0.5 micron of pressure.Aluminum is then vaporized from a suitably heated surface to whichmetallic aluminum is supplied. Alternatively, the aluminum may bevaporized by means of an electron beam, e.g. utilizing electron-beamvaporizing apparatus and techniques as known in the art, wherein thealuminum source is positioned in a vacuum chamber in the path of a beamof electrons from a suitable electron source with appropriate voltageapplied between the electron source and the aluminum source, so that theelectron beam vaporizes aluminum from the latter source for depositionon the substrate to be coated.

An alternative example of a vacuum-metalizing technique suitable fordepositing aluminum from vapor on the zinc-coated steel surface in thepresent method is that described in United States Patent No. 3,117,887,wherein a strip of sheet metal substrate to be coated is continuouslyadvanced through a vacuum chamber past a vertical plate or block whichis maintained at aluminum-vaporizing temperature by electricalresistance heating, i.e. by passage of electric current therethrough.Aluminum in wire form is fed into contact with the upper part of avertical surface of the heated block (such surface being parallel to andfacing the metal substrate surface to be coated); the aluminum melts andflows down over the vertical block surface and evaporates therefrom as avapor which deposits on the substrate.

The product of the invention, as obtained by the described method, is azinc-coated steel article (of sheet, strip or other form) having a thin,continuous vapordeposited outer coating of aluminum. The integrity ofthe zinc coating is maintained through the aluminum deposition step;i.e. in the final product the zinc layer has integrity as a zinc coatingand indeed is itself a conventional adherent coating of zinc asheretofore employed for protection of steel surfaces against corrosion.The vapordeposited aluminum coating overlies this zinc coating andadheres thereto. A presently preferred range of values of thickness forthe vapor-deposited aluminum overlay is between about 5 microinches andabout 100 microinches, a thickness of at least about microinches beingpresently considered particularly advantageous and a range of betweenabout and about 60 microinches being especially preferred. The aluminumcoating thickness may exceed 100 microinches if desired, the lattervalue representing a preferred upper limit from the standpoint ofeconomy.

After the zinc and aluminum coatings have been applied as describedabove, the coated steel surface may, if desired, optionally be treatedwith a chromite solution, in accordance with conventional procedures aspresently used for treating zinc coatings to prevent white rust, and fortreating aluminum coatings to promote paint adherence. Many suchchromate solutions are well-known in the art and commercially available;by way of specific example, the zinc-coated steel surface bearing thealuminum overlay may be treated with Iridite Solution 149, which ismanufactured by Allied Research Products, Inc., and is believed to bebasically a potassium dichromate solution. This chromate treatment incertain instances appears to enhance the corrosion resistance of thecoated surface to some extent, but is not necessary to the attainment ofthe advantages of the invention and thus may be omitted.

The effect of the aluminum overlay on the corrosionresistant propertiesof zinc-coated steel has been ascertained by testing, for resistance tocorrosion under exposure to salt spray, zinc-coated steel panels(including both hot-dip galvanized and electroplated zinc-coated panels)prepared and coated with a vapor-deposited overlay of aluminum inaccordance with the invention. The samples tested included panels havingaluminum coatings 15, 30, and 60 microinches thick, deposited over zinccoatings of various thicknesses ranging from less than microinches tomore than 950 microinches. Some of these panels were treated, i.e.before testing, with a chromate solution as described above; others wereuntreated. Results of the tests were compared with results of similartests on panels having conventional hot-dip galvanized or electroplatedzinc coatings of like thickness but no aluminum overlay.

The testing procedure used was a standard ASTM test method(specification No. 8117-62) for resistance to corrosion under saltspray, wherein the panel under test is continuously exposed to saltspray until red rust develops on the panel surface, the resistance tocorrosion being determined by the duration of exposure and theproportion of the panel surface covered with red rust as appraised byvisual inspection. In the present tests, each of the panels was exposedto salt spray until 10% of the panel surface was covered with red rust;the exposure time required to produce this 10% red rust condition servedas a measure of the resistance of the panel to corrosion.

These tests demonstrated that for all the thicknesses of zinc andaluminum coatings tested, the presence of the aluminum overlay greatlyincreased the time required for zinccoated steel panels to reach 10%coverage with red rust under exposure to salt spray, as compared withpanels having zinc coatings of the same thickness but no coating ofaluminum. Thus, for panels without aluminum overlay having zinc coatingsup to about 175 microinches thick, the exposure time required to exceed10% red rust was well under hours, and ordinarily less, i.e. 50 hours orso. In contrast, a series of 18 panels havin zinc coatings in the samethickness range with aluminum overlays 15 to 60 microinches thick allrequired more than 100 hours to reach 10% red rust, and a majority ofthese samples required more than 200 hours, even in some cases above 350hours, to reach 10% red rust. Again, in the case of panels Withoutaluminum overlay having zinc coatings 300 to 700 microinches thick, theexposure times necessary to attain 10% red rust were less than 200hours; a series of 18 panels having zinc coatings in the latterthickness range and aluminum coatings 15 to 60 microinches thick allrequired more than 300 hours of exposure to reach 10% red rust, and for15 of these panels having aluminum overlays 30 to 60 microinches thickthe exposure times to 10% red rust ranged from 600 to 1,200 hours. Thelongest exposure times (900 1,200 hours) in this series were achievedwith panels having a zinc thickness of between about 450 and 650microinches and an aluminum thickness of 30 to 60 microinches, treatedwith chromate before testing, although exposure times well above 600hours were also achieved With panels having an aluminum thickness (overzinc) in the latter range, Without chromate treatment.

It will therefore be appreciated that a principal advantage of thepresent invention is in greatly enhancing the protection againstcorrosion afforded by zinc coatings on steel. As shown in the foregoingtests, by the provision of a thin (5-100 microinch) vapor-depositedaluminum overlay on a zinc coating in accordance with the invention theduration of effective protection afforded by the coating can beincreased by 100 to several hundred hours, i.e. in many cases by afactor of two, three or even more over the duration of protectionafforded by zinc coatings Without aluminum overlay. The remarkableenhancement of corrosion resistance of zinc-coated steel thus achievedis not fully explained, especially since vapor-deposited aluminum aloneon a bare steel substrate affords very little protection against saltspray; for example, steel having a 30-microinch vapor-deposited aluminumcoating (with no zinc coating) requires only about 10 hours of exposureto salt spray to reach 10% red rust. In other Words, then, the saltspray resistance of zinc-coated steel with a vapor-deposited aluminumouter coating is much more than a mere additive function of theprotection aiforded by either coating alone.

While aluminum layers 15 to 60 microinches thick were used in theforegoing tests, and represent presently preferred values of thicknessfor the aluminum overlay on the zinc-coated articles of the invention,the advantages of the invention may be realized with even thinnercoatings of aluminum. For example, steel panels coated with 15 and 30microinches of zinc and further coated with a.

vapor-deposited flash of aluminum (3 microinches) were found to exhibitimproved resistance to corrosion in a humidity cabinet, over samplescoated with an equal thickness of zinc but having no aluminum overlay.

Although the method of the invention has been described above asincluding the application of a zinc coating by conventionalelectroplating or hot-dip galvanizing procedures, other operations forapplying zinc coatings to steel surfaces may also be employed, followedby vapor-deposition of aluminum, with like advantages in improving thecorrosion resistance of such zinc coatings. For example, the zinccoating may itself be applied to the steel surface by vapor deposition.One especially elfective procedure for depositing zinc on steel fromvapor is described in the copending application of the presentapplicants, Ser. No. 423,249, filed Jan. 4, 1965, and now abandoned. Inthe latter procedure, the steel surface (after appropriate cleaning) isfirst coated with a thin adherent layer of copper or other metalselected from a specific class of metals in the upper range of theelectromotive force series, namely gold, silver and copper, brass beingalso deemed a member of the named group in that the latter is an alloyconsisting essentially of copper and zinc wherein copper predominates.This coating may be applied e.g. by ionic deposition from an aqueousmedium or by electroplating and is followed by a vacuum metalizingdeposition of zinc from vapor on the treated surface. Thefirst-mentioned coating serves to enhance the adherence of thevapor-deposited zinc, which may be applied for example by directing zincvapor onto the surface in a vacuum chamber. The zinc vapor may beproduced in any convenient manner, for example by electronbeamvaporization as described above for vaporization of aluminum.

In the use of this latter procedure in the method of the presentinvention, the steel article bearing the vapordeposited zinc'coating issubjected to a further vacuum metalizing step for deposition of aluminumfrom vapor to provide an aluminum overlay, in the same manner asdescribed above. Again, this aluminum overlay is found to afford theaforementioned advantages in providing far superior resistance tocorrosion of the .vapor deposited zinc coating.

The accompanying drawing shows, for illustration of the novel productsand also to indicate the nature of the process, a diagrammatic,perspective, enlarged view of a portion of a steel sheet produced with azinc coating on both sides in accordance with the invention.

Thus, in the drawing, the body of the steel sheet 10, which may be coldrolled strip, is first coated, e.g. by hot-dip galvanizing,electroplating, or vapor deposition, with an adherent layer of zinc ll,11' on its opposite faces. Thereafter, aluminum coatings 12, 12' areapplied by vapor deposition of aluminum in a vacuum. The zinc layer maybe, for example, from 20x10" inch to 1,500 inch or more in thickness,while the aluminum overlay may be, for example from 5 10- inch to 1001()- inch in thickness. If desired, one of the aluminum coatings may beomitted, as by simply directing vapor onto one face of the sheet duringits passage through the vacuum chamber, and also one of the zinccoatings may be omitted, to provide a steel sheet having only one coatedsurface. Again, the sheet may be coated with thicker coatings on oneside than on theother for use where the two surfaces of the product areexposed to dilfering corrosion conditions, e.g. as in automobile bodieswherein one side of the sheet is painted and the other is exposed tocorrosive material such as salt splashed up from roadways.

By way of further and more specific illustration of the inventionreference may be had to the following specific examples:

Example I Each of a series of cold-rolled steel panels was alkalicleaned in a solution containing 40 g./liter of sodium carbonate, 30g./liter of trisodium phosphate and 20 g./liter of sodium hydroxide. Thetemperature of the cleaner was maintained at 200 F. and the steel panelswere cleaned cathodically at 50 amperes per square foor for one minute.The panels were then rinsed, pickled in muriatic acid for 30 seconds,rinsed again and zinc plated in a bath containing g./liter of zinc (assulfate) and 30 g./liter (NHQ $0 the bath having a pH of 3.0 and atemperature of 80 F., at a current density of 2t) amp./ft. Plating timeswere adjusted to give zinc coatings of 25, 50, and microinches ondifferent panels. After zinc plating the samples were rinsed and dried.Thereafter, some of the samples were placed in a vacuum chamber, the airwas evacuated, and these samples were plated with vapor depositedaluminum. The aluminum was deposited under a vacuum (i.e. pressure) of0.1 micron by placing an aluminum clip on a tungsten filament andheating the filament to vaporize the aluminum, the aluminum beingevaporated at a temperature of about 1370 C. The thickness of thealuminum overlay thus produced, on various samples, was 15 and 30microinches.

In addition, several samples were prepared in which aluminum vapor wasdeposited directly on bare steel, with no zinc coating.

Samples of the zinc-coated steel without aluminum overlay, of thealuminum-coated steel without zinc and of the aluminum over zinc onsteel were placed in a salt spray atmosphere and tested to determinetheir resistance to corrosion by duration of exposure and extent of redrust produced. Results are summarized in the following table:

As indicated in the table, samples of zinc coated steel without aluminumoverlay having zinc thickness up to microinches reached a condition of10% or greater coverage with red rust in six hours, or 20% in twelvehours. Samples of aluminum-coated steel without zinc reached a conditionof 25% to 50% red rust coverage in six to twelve hours. However, thesamples prepared in accordance with the present invention, having analuminum overlay on a zinc coating, attained only 10% to 20% red rustcoverage in 48 to 114 hours, i.e. these latter samples exhibited verymarkedly improved resistance to corrosion. Thus, whereas a Zinc coatedpanel without aluminum having zinc thickness of 165 microinches reached20% red rust in 12 hours, and an aluminum coated panel without zinchaving an aluminum thickness of 15 microinches reached 50% red rust insix hours, a panel coated with 150 microinches of zinc and 15microinches of vapor deposited aluminum (for a total coating thicknessof 165 microinches) reached only 20% red rust in 48 hours. Again, apanel having a 100 microinch zinc coating and a 30 microinch aluminumcoating reached only 10% red rust in 114 hours whereas a panel having a115 microinch thick zinc coating without aluminum reached 10% red rustin six hours and a panel bearing 30 microinches of aluminum without zincreached 25% red rust in twelve hours.

Example II Five zinc-coated steel panels prepared by hot dip galvanizingand having a zinc coating thickness of 800 microinches were cleaned withan alkaline scrubber-type cleaning line and thereafter further coatedwith 45 microinches of aluminum by vapor deposition over the zincsurfaces. The aluminum was deposited under a vacuum (i.e. pressure) of0.1 micron at a rate of 0.03 lb./ft. per minute, the aluminum beingevaporated at a temperature of about 1370 C. These samples and fivesimilar steel panels having hot-dip galvanized zinc coatings of the samethickness but no aluminum overlay, were then exposed to salt spray until10% red rust coverage was attained. For the five galvanized panelswithout aluminum overlay, the average exposure time required to reach10% red rust was 264 hours; for the five panels having the aluminumlayer, the average time required to reach 10% red rust condition was 840hours.

We claim:

1. A method of producing a zinc-coated steel article with a vapordeposited aluminum overlay, comprising applying to a steel surface anadherent coating of zinc, and thereafter applying a coating of aluminumto the zinccoated steel surface by depositing aluminum vapor thereon.

2. A method according to claim 1, wherein said step of applying saidZinc coating comprises electroplating zinc on the steel surface.

3. A method according to claim 1, wherein said step of applying saidzinc coating comprises immersing said steel surface in molten zinc.

4. A method according to claim 1, wherein said step of applying saidzinc coating comprises directing zinc vapor onto said steel surface toestablish an adherent vapor-deposited zinc coating thereon.

5. A method of producing a zinc-coated steel article with a vapordeposited aluminum overlay, comprising applying to a steel surface anadherent coating of zinc and thereafter applying a coating of aluminumto the zinccoated steel surface by depositing aluminum vapor thereon insufficient amount to provide an aluminum coating of at least about 5 10inch in thickness.

6. A method according to claim 5 wherein said zinc coating has athickness of at least about 20 X 10 inch.

7. A method according to claim 5, wherein said aluminum coating isbetween about 10- inch and about 100x10 inch in thickness.

8. A method of producing a zinc-coated steel article with a vapordeposited aluminum overlay, comprising applying to a steel surface anadherent coating of zinc to a thickness of at least about 10 inch, andthereafter applying a coating of aluminum to the zinc-coated steelsurface by depositing aluminum vapor thereon in vacuum in sufiicientamount to produce an aluminum coating of between about 5X10 inch andabout x10" inch in thickness.

9. A Zinc-coated steel article with a vapor deposited aluminum overlay,consisting essentially of a steel base having a surface carrying a layerof zinc in adherence to said steel surface, said zinc layer havingintegrity as a zinc coating upon said steel surface, and an adherentcoating of aluminum applied over said zinc coating by vapor deposition.

10. A zinc-coated steel article with a vapor deposited aluminum overlay,consisting essentially of a steel base having a surface carrying a layerof zinc, having a thickness of at least about 20X 10- inch, in adherenceto said steel surface, said zinc layer having integrity as a zinccoating upon said steel surface, and an adherent coating of aluminumapplied over said zinc coating by vapor deposition.

11. A zinc-coated steel article with a vapor deposited aluminum overlay,as defined in claim 10, in which the aluminum coating has a thickness ofat least about 5 X10" inch.

12. A zinc-coated steel article with a vapor deposited aluminum overlay,as defined in claim 10, in which the aluminum coating has a thickness inthe range of about 15 10- inch to about 100 10 inch.

References Cited UNITED STATES PATENTS 2,405,662 8/1946 McManus et a1.117-107.1 X 2,490,978 12/ 1949 Osterheld. 2,566,138 8/1951 Osterheld.2,959,494 11/1960 Shepard 117107 X 3,117,887 1/1964 Shepard et a1117-l07.1

FOREIGN PATENTS 400,752 11/1933 Great Britain.

ALFRED L. LEAVITT, Primary Examiner.

J. R. BATTEN, JR., Assistant Examiner.

US. Cl. X.R.

